Recovering base oil from contaminated invert emulsion fluid for making new oil- /synthetic-based fluids

ABSTRACT

Base oil can be recovered from contaminated O/SBF by combining a chemical process with a mechanical process. The chemical treatment includes adding a demulsifier, an anionic surfactant, a non-ionic surfactant and/or a mutual solvent to the contaminated O/SBF in an amount effective to separate the base oil from the contaminated O/SBF fluid followed by mechanical separation of oil from water, and optionally from any solids present. The recovered base oil (i.e. conventional drilling fluid, conductive drilling fluid and constant rheology drilling fluid, etc.) may then be reformulated to make a new OBM of the same type from which the base oil was recovered, or as a fuel for engines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application from U.S. patentapplication Ser. No. 15/189,665 filed Jun. 22, 2016 and issued as U.S.Pat. No. ______ on ______, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/184,469 filed Jun. 25, 2015, all of whichare incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to methods for recovering base oils fromcontaminated invert emulsion drilling fluids, and in another onenon-limiting embodiment, relates to methods for recovering base oilsfrom contaminated invert emulsion drilling fluids and re-using them ininvert emulsion drilling fluids of the same type from which they wererecovered or as a fuel source.

BACKGROUND

Drilling fluids used in the drilling of subterranean oil and gas wellsalong with other drilling fluid applications and drilling procedures arewell known. In rotary drilling there are a variety of functions andcharacteristics that are expected of drilling fluids, also known asdrilling muds, or simply “muds”.

Drilling fluids are typically classified according to their base fluid.In water-based muds, solid particles are suspended in water or brine.Oil can be emulsified in the water which is the continuous phase.Brine-based drilling fluids, of course are a water-based fluid (WBF) inwhich the aqueous component is brine. These may also be calledwater-based muds (WBMs). Oil-based fluids (OBFs) are the opposite orinverse. Solid particles are often suspended in oil, and water or brineis emulsified in the oil and therefore the oil is the continuous phase.Oil-based muds (OBMs) can be either all-oil based or water-in-oilmacroemulsions, which are also called invert emulsions. In oil-based mudthe oil can consist of any suitable oil that may include, but is notlimited to, diesel, mineral oil, synthetic oil, esters, or olefins. OBFsas defined herein also include synthetic-based fluids or muds (SBFs orSBMs) which are synthetically produced rather than refined fromnaturally-occurring materials. SBFs often include, but are notnecessarily limited to, olefin oligomers of ethylene, esters made fromvegetable fatty acids and alcohols, ethers and polyethers made fromalcohols and polyalcohols, paraffinic, or aromatic hydrocarbons, alkylbenzenes, terpenes and other natural products and mixtures of thesetypes.

Inventory management of contaminated O/SBF is one of the biggestchallenges in the oil and gas industry. When invert-emulsion drillingfluids are used during a drilling operation, they accumulatecontaminants such as drill solids (up to about 20 vol %) and in someinstances they also collect formation water. These contaminants havedrastic effect on the properties of the drilling fluids. The most commonmethod to reduce the effect of contaminants is to dilute the drillingfluid with base oil. However, this is not the ideal solution since itcontinues to increase the inventory. It should be understood that theterm “contaminated O/SBF” includes, but is not necessarily limited toused OBFs and/or SBFs that has been used at least once as a drillingfluid, completion fluid, workover fluid, stimulation fluid, productionfluid or some other oilfield or non-oilfield use, such as a metalcutting fluid.

It would be desirable if the base oil from contaminated O/SBF containingcuttings, drill solids, formation water and formation fluids could berecovered and reused in order to minimize growing the inventory.

SUMMARY

There is provided, in one non-limiting form, a method of recovering baseoil from a contaminated O/SBF comprising of solids and water in oilemulsion, where the method comprises in this sequence: mixing at leastone chemical with the contaminated O/SBF, where at least one chemical isselected from the group consisting of demulsifiers, anionic surfactants,non-ionic surfactants, mutual solvents, microemulsions, and combinationsthereof, and where the chemical is mixed in an amount effective to breakthe water and oil emulsion; and the method further comprisesmechanically separating the base oil, water and solids into three phasesusing a mechanical separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graph illustrates the oil-water ratio of a contaminated O/SBF,along with the oil-water ratio of the recovered oil as a function ofchemical dosage added to the O/SBF;

FIG. 2 graph illustrates the total amount of retained solids in therecovered base oil in vol % after the indicated dosages of chemical wasadded to the contaminated O/SBF of FIG. 1; and

FIG. 3 is a schematic illustration of one non-limiting embodiment of abase oil recovery system in accordance with the method described herein.

DETAILED DESCRIPTION

The base oil is the most expensive component of invert emulsion fluidsper barrel. The inventive method provides the ability to recover baseoil from contaminated O/SBF that has undesired properties to use therecovered base oil in new fluids. It has been discovered that base oilcan be recovered from contaminated O/SBF using a process having at leastone chemical step followed by at least one mechanical step. In brief,the chemical treatment includes adding an ionic surfactant, an anionicsurfactant, or a non-ionic surfactant, microemulsion system, and/or oneor more mutual solvent or a combination of these in an amount effectiveto break the emulsions in the fluid prior to mechanical separation ofwater, oil and solids into three distinct phases. Drilling fluidssystems such as conductive oil base mud, constant rheology mud systemsand/or mud systems for conventional land drilling can be made fromrecovered base oil of incumbent drilling fluid systems in the inventory.As noted, inventory management of contaminated O/SBF is one of thebiggest challenges in the oil and gas industry. The recovered base oilmay be advantageously used to formulate a new O/SBF of the same typefrom which the recovered base oil was separated or recovered.Additionally or alternatively, the recovered oil can also be used as asource of fuel for engines.

In more detail, the base oil in the contaminated O/SBF is selected fromthe group consisting of diesel, synthetic base oil, mineral base oil,paraffin and combinations thereof. Further, as defined here, the O/SBF(whether or not contaminated) includes, but is not necessarily limitedto, production fluids, drilling fluids, completion fluids, stimulationfluids, and workover fluids, particularly those used in the explorationand discovery of hydrocarbons (e.g. crude oil, natural gas, etc.), butalso other working fluids, such as metal cutting fluids and the like.

A non-limiting first step involves mixing at least one chemical into acontaminated O/SBF, where the chemical is selected from the groupconsisting of demulsifiers, anionic surfactants, non-ionic surfactants,mutual solvents, and combinations thereof, and where the chemical ispresent in an amount effective to separate the base oil from the water.The effective amount of the chemical used in the contaminated O/SBFranges from about 0.5 independently to about 8.0 vol % chemicalconcentration; alternatively ranges from about 2 independently to about5 vol %. This concentration range (which represents the total chemicalconcentration) is used when more than one chemical is present in thechemical treatment process. However, when a single chemical is usedindependently in the chemical treatment process, about 1.0 independentlyto about 5.0 vol % chemical concentration is the range specified;alternatively ranges from about 2 independently to about 3 vol %. Whenthe term “independently” is used with respect to a range, any lowerthreshold may be used together with any upper threshold to give asuitable alternative range.

Further, if the chemical is an anionic surfactant, it is an anionicsurfactant having a head group including, but not necessarily limitedto, sulfate, sulfonate, phosphate, carboxylate, and combinationsthereof. If the chemical is a nonionic surfactant, it may be selectedfrom the group consisting of alkoxylated linear alcohols, fatty acidesters, alkylpolyglucosides, alkoxylated alkyl phenols, and combinationsthereof. The chemical can also consist of microemulsion systems,selected from a group of anionic, nonionic, cationic and mixturesthereof. Microemulsions are defined herein as an emulsion that is inthermodynamic equilibrium consisting of an isotropic liquid mixture ofoil, water and surfactant, frequently in combination with aco-surfactant. If the chemical is a mutual solvent, it may be selectedfrom the group consisting of glycol ethers, alcohols, and combinationsthereof. In one non-limiting embodiment, suitable chemicals include, butare not necessarily limited to, dimethyl ester, a combination ofbenzenesulfonic acid and hexadecyl (sulfophenoxy)-, disodium salt andmicroemulsions based on dimethyl 2-methylglutarate.

After adding one or more of the above-noted chemicals to the O/SBF, theresulting treated fluid is subjected to physically separating, in onenon-limiting embodiment using a three-phase centrifuge to recover thebase oil. The physically separating in this step may be by centrifuging,in a non-limiting embodiment by using a three-phase centrifuge to removethe base oil, water and any solids into separate streams. Ifcontaminated O/SBF contains a high degree of solids concentration, thisstep may be conducted in one or two stages. Most contaminated O/SBF isexpected to contain solids of some type. In the first stage, the solidsconcentration is being reduced to at least 10 vol % from thecontaminated O/SBF by using a two-phase centrifuge, followed by a secondstage of separating the oil from the contaminated O/SBF. The separatorsused in each of the mechanical separation steps may be centrifuges ordecanters.

In one non-limiting embodiment, the recovered base oil has an oil/watervolume ratio of 95/5 or greater and/or less than 2 vol % low gravitysolids (LGS) with a feeding flow rate of the treated mud going into thethree-phase separator greater than 13 gallons (49 liters) per minute. Inan alternate nonrestrictive version, the recovered base oil has anoil/water volume ratio of 98/2 or greater and/or less than 2.5-3.0 vol %LGS. Alternatively, the recovered base oil has less than 5 vol % lowgravity solids. Low gravity solids are defined as those having a lowerdensity than barite or hematite.

In one non-limiting embodiment the method has a feed rate of about 13 toabout 15 gallons per minute (about 49 to about 57 liters per minute),the method has a recovered base oil rate of from about 10 to about 12gallons per minute (about 38 to about 45 liters per minute), and themechanically separated solids are solids remaining in the recovered oilphase having an average particle size between about 3 and about 98microns. This is an unusual and unique set of process parameters.Alternatively, the method has a feed rate of 15 gallons per minute(about 57 liters per minute), a very low solids concentration in therecovered oil of about 1.26 vol % low gravity solids. In onenon-limiting embodiment, the solids remaining in the recovered oil phasehas the following distribution:

D10=3.2 microns

D50=16.3 microns

D90=97.4 microns.

In another non-limiting embodiment the distribution may be in thefollowing ranges:

D10=2.5-3.5 microns

D50=10-20 microns

D90=80-110 microns

In a different non-restrictive version the distribution may be thefollowing:

D10=4 microns

D50=18 microns

D90=10 microns.

It will be appreciated that the method herein may additionally includean option of reusing the recovered base oil in an application by eitherformulating a new O/SBM of the same type (for example, recovered dieselbase oil can be used to make a diesel-based invert emulsion fluid, andsynthetic base oil may be used to make synthetic-based invert emulsionfluid, etc.). New types of mud systems may also be formulated including,but not necessarily limited to, conductive oil base mud or constantrheology oil base mud systems that requires the same type of base oil asthe oil recovered from the mud. Combusting the recovered base oil in anengine would also be an option. However, if the recovered base oil doesnot meet the specifications for an engine, it can be used as fuel sourceand combusted for other uses, or the recovered base oil can find otherutilities, for instance as a metal cutting oil.

Examples of suitable organic materials for the oil of such system fluidsinclude but are not necessarily limited to olefins, olefin oligomers ofethylene, water insoluble esters (such as those made from vegetablefatty acids and alcohols), ethers and polyethers made from alcohols andpolyalcohols, paraffinic or aromatic hydrocarbons, alkyl benzenes,terpenes and other natural products and mixtures of these types, waterinsoluble polyglycols, diesel, water insoluble Fischer-Tropsch reactionproducts, and other organic materials, in one non-limiting embodimentmaterials that are non-toxic at the concentrations used, andcombinations thereof. Suitable olefins are branched and/or linear andpreferably are relatively non-toxic synthetic olefins. Examples ofsuitable olefins include but are not necessarily limited topolyalphaolefins, linear alpha olefins, and internal olefins, typicallyskeletally isomerized olefins. Most preferred olefins are described inU.S. Pat. Nos. 5,605,872 and 5,851,958, incorporated herein by referencein their entirety. Preferred paraffins are described in U.S. Pat. No.5,837,655, incorporated herein by reference in its entirety. Theseolefins may include olefins having from about 14 to about 30 carbonatoms, including linear alpha-olefins having at least 16 carbon atoms.C20 is about the maximum length on the isomerized olefins currentlyemployed as base oils, and the carbon number for some olefins is lower.

The used O/SBFs as well as the newly formulated O/SBFs may containconventional additives, or additives developed in the future, for thesedrilling fluids including, but not necessarily limited to, solidweighting agents, lost circulation additives, filtration controladditives, viscosifiers or rheology modifiers, emulsifiers, oil-wettingagents, lubricants, clays, tracers, brines, salts, alkalinity agents,acid gas scavengers, sweep materials, fluid loss control agents, and thelike and combinations thereof. Suitable solid weighting agents include,but are not necessarily limited to, barite, hematite, calcium carbonate,siderite, ilmenite, heavy brines, galena, and the like and combinationsthereof. Suitable lost circulation or fluid loss control additives,include, but are not necessarily limited to, lignite, asphalts,gilsonites, synthetic polymers and the like. Suitable viscosifiersinclude, but are not necessarily limited to, organophilic bentonite,attapulgite, sepiolite and dimeric and trimeric fatty acids,imidazolines, amides, synthetic polymers, and the like and combinationsthereof. These conventional solid additives may be removed using themethods of solids separation described herein, which may also removedrill cuttings that are contained in the contaminated O/SBF.

The invention will now be described with respect to some examples thatare not intended to limit the method, but to simply providenon-restrictive examples of how the method may be implemented.

Example 1

An initial contaminated O/SBF was used having an 80/20 oil/water ratio(OWR). Chemical dosages in the amount of 1.0-2.5 vol % were mixed intothe contaminated O/SBF, and then the mixtures were subjected tomechanical separation using a three-phase centrifuge. The chemical usedwas dimethyl 2-methylglutarate, The OWRs resulting for each dosage arepresented in FIG. 1. It may be seen that as more chemical was added,less water resulted in the recovered base oil.

Furthermore, the total solids remaining in the recovered base oil as avolume percent for each of the four chemical dosages is presented inFIG. 2, where the recovered base oil has been subjected to the chemicaltreatment and mechanical separation steps described herein. The % solidsare a combination of low gravity solids (LGS), high gravity solids (HGS)and soluble salts. Again, it may be seen that the vol % solids decreaseswith increasing amounts of chemical added.

Shown in FIG. 3 is a schematic illustration of one non-limitingembodiment of a base oil recovery system 10 in accordance with themethod described herein. Contaminated O/SBF is fed from holding tank forinitial contaminated O/SBF (no chemicals are added into this tank) 12via centrifugal pump 14 to one of two mixing tanks 16 or 18 via line 20(from initial contaminated O/SBF holding tank). Two mixing tanks 16 and18 are used to facilitate a continuous process. While one of the tanks16 or 18 is being filled with contaminated O/SBF, the other tank 18 or16 is in the process of mixing at least one chemical with thecontaminated O/SBF. Mixing tanks 16 and 18 have a line 22 establishingcommunication between them; line 22 is for equalizing, and in case ofoverflow from one tank to the other.

First “tote tank” 24 contains a first chemical and second “tote tank” 26contains a second chemical (in the optional embodiment where twochemicals are used) which are fed to mixing tanks 16 and 18 viaelectrical diaphragm adjustable pumps for chemical injection 25 and 27,respectively, and lines 28. Mixing tanks 16 and 18 thoroughly mix thefirst and second chemicals with the contaminated O/SBF to form amixture. Transfer line 32 may recycle the mixture via centrifugal pump30. The fluid in transfer line 32 is a recycled fluid being reprocessed(fluid is sent through two-phase centrifuge, then the same fluid withthe addition of chemical(s) is sent through the three-phase centrifuge)and then to the recovered oil tank 64. The mixture is fed viacentrifugal pump 34 and feed line from mixing tanks 16, 18 and feed line35 through flow meter 36 which regulates flow of the mixture totwo-phase centrifuge 38 (e.g. a decanter or centrifuge). As previouslynoted, it is expected that most contaminated O/SBF will contain solids,thus two-phase centrifuge 38 separates solids, for instance bycentrifugation, to give solids discharge stream 40 from two-phasecentrifuge 38 and water and oil emulsion stream 42 which goes tooptional holding/mixing tank 44, which stream is pumped via centrifugalpump 46 through flow meter 48 through line 50 to three-phase centrifuge52, although a portion of the water and oil emulsion stream may berecycled to mixing tanks 16 and/or 18 via transfer line 54.

The mixture may optionally bypass two-phase separator 38 and go directlyto three-phase centrifuge 52 via bypass line 56 in the event that themixture has reduced, minimal or essentially no solids.

Three-phase centrifuge 52 (e.g. decanter or centrifuge) separates themixture and/or the water and oil emulsion into solids discharge streamfrom three phase centrifuge 58, recovered base oil stream 60 and processwater (heavy phase) stream 62. Recovered base oil stream 60 istransferred to recovered oil tank 64 which collects the recovered oilfrom the process from where it is pumped to reserve tanks (not shown)via recovered base oil centrifugal pump 66. A portion of this oil may bepassed to mixing tanks 16 and 18 through flow meter 68 and base oilrecycle line 70. Process water (heavy phase) stream 62 is transferred tooil skimmer tank 72 for further separating the base oil 74 (by way ofskimming discharge), processed water discharge 76 transferred viacentrifugal pump 78 and solid discharge line 80 transferred via pump 82.Recovered base oil stream 60 and separated base oil (by way of skimmingdischarge) 74 gives the recovered base oil of the method describedherein.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been suggested aseffective in providing effective methods for recovering base oils fromcontaminated O/SBF. However, it will be evident that variousmodifications and changes may be made thereto without departing from thebroader scope of the invention as set forth in the appended claims.Accordingly, the specification is to be regarded in an illustrativerather than a restrictive sense. For example, specific contaminatedO/SBF, chemicals, demulsifiers, anionic surfactants, non-ionicsurfactants, mutual solvents, and proportions thereof, differentchemical treatments, and different mechanical separations falling withinthe claimed parameters, but not specifically identified or tried in aparticular method to improve recover base oils, are anticipated to bewithin the scope of this invention.

The present invention may suitably comprise, consist or consistessentially of the elements disclosed and may be practiced in theabsence of an element not disclosed. For instance, in a method ofrecovering base oil from a contaminated O/SBF comprising solids, and abase oil and water in a water and oil emulsion, the method may comprise,consist essentially of or consist of, in this sequence, mixing at leastone chemical with the contaminated O/SBF, where the at least onechemical is selected from the group consisting of demulsifiers, anionicsurfactants, non-ionic surfactants, mutual solvents, microemulsions, andcombinations thereof, and where the chemical is present in an amounteffective to break the oil and water emulsion; and mechanicallyseparating the base oil from the water and solids using a mechanicalseparator giving recovered base oil.

As used herein, the terms “comprising,” “including,” “containing,”“characterized by,” and grammatical equivalents thereof are inclusive oropenended terms that do not exclude additional, unrecited elements ormethod acts, but also include the more restrictive terms “consisting of”and “consisting essentially of” and grammatical equivalents thereof. Asused herein, the term “may” with respect to a material, structure,feature or method act indicates that such is contemplated for use inimplementation of an embodiment of the disclosure and such term is usedin preference to the more restrictive term “is” so as to avoid anyimplication that other, compatible materials, structures, features andmethods usable in combination therewith should or must be, excluded.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, relational terms, such as “first,” “second,” “top,”“bottom,” “upper,” “lower,” “over,” “under,” etc., are used for clarityand convenience in understanding the disclosure and accompanyingdrawings and do not connote or depend on any specific preference,orientation, or order, except where the context clearly indicatesotherwise.

As used herein, the term “substantially” in reference to a givenparameter, property, or condition means and includes to a degree thatone of ordinary skill in the art would understand that the givenparameter, property, or condition is met with a degree of variance, suchas within acceptable manufacturing tolerances. By way of example,depending on the particular parameter, property, or condition that issubstantially met, the parameter, property, or condition may be at least90.0% met, at least 95.0% met, at least 99.0% met, or even at least99.9% met.

As used herein, the term “about” in reference to a given parameter isinclusive of the stated value and has the meaning dictated by thecontext (e.g., it includes the degree of error associated withmeasurement of the given parameter).

What is claimed is:
 1. A method of recovering base oil from acontaminated O/SBF comprising solids, and base oil and water in a waterand oil emulsion, the method consisting of in this sequence: mixing atleast microemulsion with the contaminated O/SBF, and where themicroemulsion is mixed in an amount effective to break the water and oilemulsion; and mechanically separating the base oil from the water andsolids using a three-phase centrifuge giving recovered base oil; andcombusting the recovered base oil as a fuel source.
 2. The method ofclaim 1 further comprising formulating a new O/SBF of the same type thatthe recovered base oil was separated from.
 3. The method of claim 1where the microemulsion concentration ranges from about 0.5 to about 8.0vol %.
 4. The method of claim 1 where the base oil in the contaminatedO/SBF fluid is selected from the group consisting of diesel, mineraloil, a synthetic base oil, esters, olefins, paraffins, and combinationsthereof.
 5. The method of claim 1 where the microemulsion comprises asurfactant selected from a group of anionic surfactants, non-ionicsurfactants, cationic surfactants, and combinations thereof.
 6. Themethod of claim 1 where the recovered base oil has an oil/water volumeratio of 95/5 or greater and less than 2 vol % low gravity solids. 7.The method of claim 1 where: the method has a feed rate of about 13 toabout 15 gallons per minute (about 49 to about 57 liters per minute);the method has a recovered base oil rate of from about 10 to about 12gallons per minute (about 38 to about 45 liters per minute); therecovered base oil has less than 5 vol % low gravity solids; and themechanically separated solids are solids remaining in the recovered oilphase having an average particle size between about 3 and about 98microns.
 8. A method of recovering base oil from a contaminated O/SBFcomprising solids, and base oil and water in a water and oil emulsion,the method consisting of in this sequence: mixing at least microemulsionwith the contaminated O/SBF, and where the microemulsion is mixed in anamount effective to break the water and oil emulsion, and where themicroemulsion comprises a surfactant selected from a group of anionicsurfactants, non-ionic surfactants, cationic surfactants, andcombinations thereof; and mechanically separating the base oil from thewater and solids using a three-phase centrifuge giving recovered baseoil; and combusting the recovered base oil as a fuel source; where thebase oil in the contaminated O/SBF fluid is selected from the groupconsisting of diesel, mineral oil, a synthetic base oil, esters,olefins, paraffins, and combinations thereof.
 9. The method of claim 8further comprising formulating a new O/SBF of the same type that therecovered base oil was separated from.
 10. The method of claim 8 wherethe microemulsion concentration ranges from about 0.5 to about 8.0 vol%.
 11. The method of claim 8 where the recovered base oil has anoil/water volume ratio of 95/5 or greater and less than 2 vol % lowgravity solids.
 12. The method of claim 8 where: the method has a feedrate of about 13 to about 15 gallons per minute (about 49 to about 57liters per minute); the method has a recovered base oil rate of fromabout 10 to about 12 gallons per minute (about 38 to about 45 liters perminute); the recovered base oil has less than 5 vol % low gravitysolids; and the mechanically separated solids are solids remaining inthe recovered oil phase having an average particle size between about 3and about 98 microns.
 13. A method of recovering base oil from acontaminated O/SBF comprising solids, and base oil and water in a waterand oil emulsion, the method consisting of in this sequence: mixing atleast microemulsion with the contaminated O/SBF where the microemulsionconcentration ranges from about 0.5 to about 8.0 vol %; and mechanicallyseparating the base oil from the water and solids using a three-phasecentrifuge giving recovered base oil having an oil/water volume ratio of95/5 or greater and less than 2 vol % low gravity solids; and combustingthe recovered base oil as a fuel source.
 14. The method of claim 13further comprising formulating a new O/SBF of the same type that therecovered base oil was separated from.
 15. The method of claim 13 wherethe base oil in the contaminated O/SBF fluid is selected from the groupconsisting of diesel, mineral oil, a synthetic base oil, esters,olefins, paraffins, and combinations thereof.
 16. The method of claim 13where the microemulsion comprises a surfactant selected from a group ofanionic surfactants, non-ionic surfactants, cationic surfactants, andcombinations thereof.
 17. The method of claim 13 where: the method has afeed rate of about 13 to about 15 gallons per minute (about 49 to about57 liters per minute); the method has a recovered base oil rate of fromabout 10 to about 12 gallons per minute (about 38 to about 45 liters perminute); the recovered base oil has less than 5 vol % low gravitysolids; and the mechanically separated solids are solids remaining inthe recovered oil phase having an average particle size between about 3and about 98 microns.